The invention relates to a method for examining at least one wafer with regard to a contamination limit, and to an EUV projection exposure system for carrying out the method.
On the surfaces of optical elements, especially multilayer mirrors, used in EUV projection exposure systems, contaminating substances are progressively deposited during operation. Those substances are produced, for example, by reaction of gases present in the vacuum environment of the optical surfaces with the EUV radiation to form low-volatility solids. Examples of gases that form such a low-volatility oxidic solid with oxidising radicals produced by the EUV radiation, such as, for example, OH., O., are sulphur, phosphorus, fluorine and silicon compounds, which may outgas from components present in the system. Under irradiation with EUV radiation, gaseous hydrocarbons, typically having a mass number of 45 and above, may also result in a layer of carbon on the optical surfaces. In particular, the contaminating substances mentioned and other contaminating substances may outgas from the wafer provided with a photosensitive layer (resist). That is particularly the case when processing defects occur during the wet-chemical application of the resist to the silicon wafer and during subsequent pre-baking. If temperature management is not optimal during pre-baking, the resist does not cure completely, leading to increased outgassing. A defective composition of the resist or the transporting of the wafer to the EUV projection exposure system may also contribute to the resist outgassing contaminating substances to a greater extent.
As a result of the deposition of those contaminating substances on the optical surfaces of the EUV projection exposure system, those surfaces may be irreversibly damaged, with in some cases enormous costs arising from the downtimes caused during replacement of damaged elements.
It is known to keep the contaminating substances outgassed from the wafer during exposure thereof away from the optical components in the projection exposure system using a stream of gas.